Project Summary/Abstract Altered division, or fission, of mitochondria has severe consequences even death. Yet the reasons for this are unknown. It is postulated that the mitochondria have their own lifecycle that involves fission of unhealthy mitochondria to remove them in an autophagic process called mitophagy. These quality control processes are evolutionarily conserved between budding yeast and humans, although extent to which has recently been called into question. In yeast, fission requires the protein FIS1 that is now thought to be dispensable for fission in mammals, but indispensable for mitophagy. Consistent with this view are FIS1 interactions in mitophagy human cell culture. Contrary to this view, loss of FIS1 elongates mitochondrial and displaces the fission mechanoenzyme DRP1 from mitochondria in some, but not all, cell types. Mutations to some evolutionarily conserved residues in FIS1 impair fission and DRP1 localization. Mutations to different conserved residues impair binding to a critical adaptor in mitophagy, the Rab7 GTPase Activating Proteins TBC1D15 and TBC1D17. These findings suggest that FIS1 is conserved for roles in both fission and mitophagy. Using yeast- inspired mutations, along with state-of-the-art genetic, microscopic, and structural tools, we are now poised to determine how conserved components govern the fate of mitochondria between fission, mitophagy, or apoptosis. To understand the protein-protein interactions that govern this, biochemical and structural studies will be integrated with state-of-the-art cell biological and genetic approaches. A better understanding of the protein machinery and how it works will identify key points of regulation that may be targeted in future studies with small molecules to inhibitor, and activate fission, mitophagy, and apoptosis. The discovery of such molecules may ultimately lead to treatments for diseases in which enhanced, or impaired, fission activity is central.